1. Field of the Invention
This invention relates to radio frequency transceiver design, and, more particularly, to the design of a class D amplifier in a radio frequency transmitter.
2. Description of the Related Art
Radio frequency (RF) transmitters/receivers are used in a wide variety of applications, including wireless network interfaces, mobile telephones, and Bluetooth interfaces. RF transceivers also feature prominently in wireless audio technology directed to headphones and earphones, home audio/theater systems and speakers, portable audio/media players and automotive sound systems. Robust, high-quality audio and low-power RF capability can make it possible for consumer and automotive original equipment manufacturers (OEMs) to integrate wireless audio technology into portable audio devices and sound systems. Overall, various RF technologies lend themselves to a number of applications in the consumer world to create high-fidelity home theater environments and distribute audio in the home and other environments.
A radio communication system typically requires tuned circuits at both the transmitter and receiver. The transmitter is an electronic device that propagates an electromagnetic signal, representative of an audio signal, for example, typically with the aid of an antenna. An RF transceiver is designed to include both a transmitter and a receiver, combined to share common circuitry, many times appearing on the same piece of Integrated Circuit (IC) chip. If no circuitry is common between transmit and receive functions, the combined device is referred to as a transmitter-receiver. RF Transceivers use RF modules for high-speed data transmission. In most systems, digital processors or processing elements (which are oftentimes software-programmable) are used to perform conversion between digital baseband signals and analog RF, and oscillators are used to generate the required periodic signals.
The receiving end of an RF transceiver system can include an audio output path to reproduce received audio signals. Audio signal reproduction typically involves amplification, which can be performed with the use of class D amplifiers configured in the audio output path. Class D amplifiers are switching amplifiers in which all power devices (e.g. MOSFETs) are operated as binary switches, which are either turned on or turned off, with preferably no time lapse during the transitions between the two states. The power stage structure of class D amplifiers is essentially identical to that of a synchronously rectified buck converter, which is a type of switching power supply. While buck converters usually function as voltage regulators, delivering a constant DC voltage into a variable load while sourcing current to the load, class D amplifiers deliver a constantly changing voltage into a fixed load, with the current and voltage independently changing signs (positive to negative and negative to positive).
Class D amplifiers are mostly used as power amplifiers intended to reproduce signals with a bandwidth well below the switching frequency. The theoretical power efficiency of class D amplifiers is 100%, that is, all power supplied to the amplifier is delivered to the load. In actuality, however, while not 100% efficient, power MOSFETs can still feature efficiencies well over 90%. The binary waveform is typically derived using pulse-width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation), sliding mode control (also referred to as “self-oscillating modulation”) or discrete-time forms of modulation, such as delta-sigma modulation. One way of creating the PWM signal is by comparing a high frequency triangular wave with the audio input, which generates a series of pulses having a duty-cycle that is directly proportional with the instantaneous value of the audio signal. The comparison output is used as a control signal for a MOS gate driver that drives a pair of high-power switches (usually MOSFETs), producing an amplified replica of the PWM signal. An output filter then removes the high-frequency switching components of the PWM signal and recovers the audio information that the speaker can use.
Most often the time resolution afforded by practical clock frequencies is only a few hundredths of a switching period, which is not enough to insure low noise. The pulse length gets quantized, resulting in quantization distortion. Furthermore, the actual output of the amplifier is not only dependent on the modulated PWM signal. Among other things, the output voltage can be directly amplitude-modulated by the power supply voltage, the output impedance may become non-linear due to dead-time errors, and the output filter can have a strongly load-dependent frequency response. One way to combat errors, regardless of their source, is negative feedback, which is implemented by creating a feedback loop that includes the output stage, using a simple integrator. The output filter may be included in the feedback loop by using a PID controller, sometimes with additional integrating terms. However, minimizing output noise and distortion remains a constant challenge.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.